The Marine Conservation Society said there had also been sightings reported on beaches in Pembrokeshire, Isles of Scilly and Ireland.

The 'Floating Terror', which is not a jellyfish but a floating colony, has long tentacles that can cause a painful sting and be fatal in extremely rare cases.

Matt Slater, a marine awareness officer at the Cornwall Wildlife Trust, said: "This is an unprecedented event and we urge the public to be cautious and to keep an eye out for unusual species being stranded.

Their tentacles can be 10 or 15 meters long and can be difficult to see (Image: Getty)

"We may see other rare warm water species washing up.

"The stings are incredibly rare and the man o' war is actually a beautiful life form, wonderfully adapted to life in the open ocean, and are only seen in extremely rare cases on our shores."

Dr Peter Richardson, head of ocean recovery at the Marine Conservation Society, added: "Portuguese man o' war are ocean-going animals, propelled by the wind on their inflatable sail as they fish the depths with their stinging tentacles.